Photo annealing effect on p-doped inverted organic solar cell

Abstract

We report the transient positive photo annealing effect in which over 600% boost of power conversion efficiency was observed in inverted organic photovoltaic devices (OPV) made from P3HT/PCBM by spray method, after 2 hrs of constant solar AM 1.5 irradiation at low temperature. This is opposite to usual photodegradation of OPV, and cannot be explained by thermal activation alone since the mere temperature effect could only account for 30% of the enhancement. We have investigated the temperature dependence, cell geometry, oxygen influence, and conclude that, for p-doped active layer at room temperature, the predominant mechanism is photo-desorption of O2, which eliminates electron traps and reduces space charge screening. As temperature decreases, thermal activation and deep trap-state filling start to show noticeable effect on the enhancement of photocurrent at intermediate low temperature (T = 125 K). At very low temperature, the dominant mechanism for photo annealing is trap-filling, which significantly reduces recombination between free and trapped carriers. At all temperature, photo annealing effect depends on illumination direction from cathode or anode. We also explained the large fluctuation of photocurrent by the capture/reemit of trapped electrons from shallow electron traps of O2- generated by photo-doping. Our study has demonstrated the dynamic process of photo-doping and photo-desorption, and shown that photo annealing in vacuum can be an efficient method to improve OPV device efficiency.

On the Control of Chromophore Orientation, Supramolecular Structure, and Thermodynamic Stability of an Amphiphilic Pyridyl-Thiazol upon Lateral Compression and Spacer Length Variation

“…We attribute this decrease in the fluorescence intensity to static quenching upon reorientation of the chromophores during fluorescence detection. This reorientation from random molecule orientations after drop casting to energetically more favorable supramolecular geometries is supposed to be driven by thermalized absorbed energy, i.e., by photoannealing . The quenching might be due to self-quenching or unfavorable rotation of the TDVs.…”

“…This reorientation from random molecule orientations after drop casting to energetically more favorable supramolecular geometries is supposed to be driven by thermalized absorbed energy, i.e., by photoannealing. 70 The quenching might be due to self-quenching or unfavorable rotation of the TDVs. Thiazole 5.…”

Photoelectric Memory Effect in an Organic Bulk Heterojunction Device

“…In another inverted heterojunction OPV, reported by Lafalce et al, 38 a slow drift in the dark I−V curve of the device was observed when the device was exposed to air. In the absence of the ZnO layer in their device, the change was related to the photodoping effect of the P3HT by reacting with oxygen and affecting the charge in the space charge region.…”

“…In addition, the transition from the rectifying property to the linear response and vanishing the hysteresis effect indicate that, at one of the interfaces, there was an energy barrier in the dark, and the energy barrier was gradually reduced upon illumination. Although, to the best of our knowledge, the optical effect of switching between the rectifying and linear characteristics has not been reported yet, the change in the I–V curve with large time constants have been experienced before in several OPV devices. − Recently, Wilken et al observed a gradual reduction of the forward current with a time constant in the order of 30 min in an ITO/ZnO nanoparticles/P3HT:PCBM/PEDOT device after exposing to ambient air. The change in the response was explained through adsorption and desorption of oxygen at the ZnO surface adjacent to the organic layer, which resulted in change of the band bending in the ZnO layer (surface state charging).…”

“…However, the existing theories developed by Wilken 36 and Lafalce 38 can be used for explaining the effect. The energy diagram of the device is presented in Figure 9.…”